Procedure and apparatus for the cleaning of flue gases containing sulfur dioxide

ABSTRACT

A process for cleaning flue gases containing ash and sulfur dioxide produced by burning sulfur-containing coal in the combustion chamber of a circulating fluidized-bed firing system includes delivering a particulate SO 2  sorbent into the combustion chamber. A mixture including portions of the ash, the reaction product produced in the reaction of the SO 2  sorbent with the sulfur dioxide, and unreacted SO 2  sorbent is fed from the combustion chamber to a mixing unit. In the mixing unit, water or an aqueous sodium-containing solution is mixed with this mixture and the unreacted SO 2  sorbent is converted into a hydration product at a reaction temperature of 60° to 100° and at atmospheric pressure. The ash, the reaction product, and the hydration product is returned from the mixing unit into the combustion chamber, and the hydration product is reactivated to an SO 2  sorbent at a combustion-chamber temperature of 700° to 950° C.

BACKGROUND OF THE INVENTION

[0001] This invention relates to a procedure as well as an apparatus forthe cleaning of flue gases containing sulfur dioxide that come fromcirculating fluidized-bed firing systems.

[0002] Circulating fluidized-bed firing systems are used in particularfor the low-emission combustion of fossil fuels, e.g. coal, peat, wood,and so forth. In burning sulfur-containing coal, for example, theoxidation of the sulfur produces sulfur dioxide, which gets into theatmosphere via the flue gas. These emissions, which are harmful to theearth's atmosphere, are returned to the earth as acid rain by way of theweather cycle. Various procedures have been developed for reducing theseharmful emissions to the greatest possible extent.

[0003] An overview of the retaining of sulfur dioxide in fluidized-bedfiring systems was presented by E. J. Anthony during the “MediterraneanCombustion Symposium” in Antalya, Turkey in June 1999.

[0004] For example, a familiar procedure is the addition of afine-grained alkaline SO₂ sorbent, generally limestone (CaCO₃), burntlime (CaO) or also dolomite, into the combustion chamber of thefluidized-bed firing system. Here, first of all the roasting (calciningprocess) of the limestone to burnt lime (CaO) takes place, andsubsequently a reaction occurs between the roasted limestone and thesulfur dioxide of the flue gas.

[0005] If in this connection the limestone is exposed to thetemperatures of 700° C. to 950° C. that are present in a circulatingfluidized-bed firing system, namely if carbon dioxide is driven off fromthe limestone, then what remains is burnt lime, which because of thedriving off of the CO₂ has a high degree of porosity and thus a highspecific surface area.

[0006] The subsequent gas-solid reaction of the burnt lime (sorbent)with sulfur dioxide and oxygen is a surface reaction, and this is whythe creation of a high specific surface area is a fundamentalprerequisite for this reaction. Remaining behind as a solid reactionproduct is calcium sulfate or gypsum (CaSO₄), which stays in the poresor on the surface of the sorbent or the burnt lime.

[0007] Depending on the grain size of the limestone or SO₂ sorbent usedand on its abrasion properties, either the aggregate of thesorbent-reaction product (lime-gypsum aggregate) remains long enough inthe combustion chamber for it to be drawn off via the components of thecombustion-chamber ash removal system, or in the case of small particlesthe sorbent-reaction product aggregate leaves the combustion chambertogether with the flue-gas stream and is subsequently separated out inthe following flue-gas filter.

[0008] The mixture composed of fuel ash, reaction product, and free,unreacted sorbent that is drawn off via the combustion-chamber ashremoval system is generally referred to as bottom ash or coarse ash. Theparticle size of this coarse ash is for the most part larger than 100μm. The maximum grain diameter can amount to several mm.

[0009] The ash carried off with the flue gas that is subsequentlyseparated out in the filter is generally called filter ash. Depending onthe quality of the cyclone/separator, the grain size of this ashencompasses the small grain fractions up to about 200 μm in diameter.

[0010] From knowledge gained by constructing fluidized-bed firingsystems, it is evident that for the degree of desulfurization requiredin industrial use, namely a reduction in sulfur dioxide of from 70% to99%, the desulfurization reaction requires a high excess of sorbent.This requirement is all the higher the greater is the demand placed onthe degree of desulfurization.

[0011] If one uses the Ca/S ratio as a measure for the added limestoneor another sorbent, namely the molar quotient of externally supplied Caand the total sulfur of the fuel that is present, then typical Ca/Svalues for fluidized-bed firing systems lie between 2 and 4 for a degreeof desulfurization of 95%.

[0012] This requirement has economic disadvantages, in particularbecause in general this raises not only the operating costs for theprocurement of limestone or of another sorbent, but also thewaste-disposal costs for the resulting ash due to the fraction ofunreacted sorbent.

[0013] In connection with the above-named desulfurization procedure forthe flue gas in a circulating fluidized-bed firing system, it has provedto be a shortcoming that the limestone or the SO₂ sorbent does not reactcompletely with the sulfur dioxide, since frequently a blanket of gypsumthat is almost gas-impermeable forms around the lime aggregate orsorbent aggregate, and also the pores of the lime or sorbent are cloggedup by gypsum as the reaction product. The physical/chemical basis forthis is the larger molar volume of SO₂ diffusing into the lime aggregateor sorbent aggregate compared to the expelled CO₂.

[0014] Especially in the interior of the grains of the sorbent-reactionproduct (lime-gypsum grains), a core of unreacted sorbent remains thatis no longer available for the reaction, since the reaction partners ofsulfur dioxide and oxygen can no longer penetrate down into this core.

[0015] The concentrations of unreacted free sorbent in the ash mixturecan be as much as 40%, both in the case of filter ash and also withcoarse ash, relative to the total ash mixture that is to be carried off.Also, within the framework of the further use of the ash mixture in thecement industry or in roadbuilding, it is desirable to have a lowerconcentration of free, namely unreacted, sorbent or limestone, to below3 to 5%.

[0016] In current fluidized-bed firing systems, various techniques arebeing used at present in order to increase the degree of utilization ofsorbents or limestone for the purposes of reducing the sulfur dioxide.

[0017] Thus, for example, in circulating fluidized-bed firing systemsthe ash accumulating in the flue-gas filter, which may still containhigh proportions of unreacted sorbent, is returned again directly intothe combustion chamber.

[0018] One drawback of this ash recycling is that the utilization of thestill free sorbent in the ash may be of only limited benefit, becausethe additional dwell time in the fluidized-bed combustion chamber issmall and because the reactivity of this ash or of the free sorbentcontained in the ash is considerably reduced compared to the originalsorbent.

[0019] Moreover, a recycling of the bed ash drawn off from thecombustion chamber is also customary. To this end, the bed ash issubjected in part to a treatment (sifting of the good grain fraction orgrinding up of the bed ash) that is aimed at increasing its degree ofreactivity. But this method as well has the drawback that its effect inreducing the requisite consumption of sorbent is very limited, since itdoes not eliminate the cause of the incomplete reaction, theabovementioned gypsum blanket around the sorbent or lime aggregate.

[0020] By way of the document U.S. Pat. No. 4,312,280, Shearer et al., asystem has furthermore been disclosed in which ash from stationaryfluidized-bed firing systems is brought into contact with water or steamand is returned to the combustion chamber of the fluidized-bed system.The mixing of the ash with steam and water takes place in a complexfluidized-bed reactor at relatively high temperatures. No reference ismade to more extensive process-technology details about operatingtemperatures of this fluidized-bed reactor or to what water admixturesare used for doing this work. This disclosed system has on the whole ahigh technological complexity and therefore has not gained muchacceptance on the market, partly also because the potential market forstationary fluidized-bed firing systems is limited to small systemsizes, and compared to circulating fluidized-bed firing systems theyhave the disadvantage of having smaller particle dwell times as well asan inhomogeneous temperature distribution.

SUMMARY OF THE INVENTION

[0021] The object of the invention, then, is to devise a procedure aswell as an apparatus for the cleaning of flue gases containing sulfurdioxide that come from circulating fluidized-bed firing systems, inwhich the above-mentioned disadvantages are avoided. Furthermore theefficiency or the degree of utilization of the sorbent used is to beincreased and thereby the quantity of sorbent needed for the sorption ofthe sulfur dioxide is to be reduced.

[0022] Through the achievement in accordance with the invention, aprocedure as well as a mechanism are provided that have the followingadvantages.

[0023] By the mixing together of the ash mixture coming from thecombustion chamber of the circulating fluidized-bed firing system (ash,reaction product, and unreacted SO₂ sorbent) with water or with anaqueous, sodium-containing solution in a mechanical mixing unit at areaction temperature of 60° to 100° C. and at atmospheric pressure, andby recycling this into the combustion chamber of the circulatingfluidized-bed firing system, the degree of utilization of the sorbent isconsiderably increased compared to a simple recycling of filter ash orbottom ash.

[0024] This effect arises from the fact that due to the mixing togetherof the ash with water or with an aqueous, sodium-containing solution ina manner corresponding the procedure in accordance with the invention,the still unreacted sorbent is first caused to react at a reactiontemperature of 60 to 100° C. with water or with an aqueous,sodium-containing solution to form a hydration product. This reaction isexothermic. The elevation in temperature as well as the reaction ratedepend on the concentration of the unreacted sorbent in the ash, thetemperature of the supplied ash, and the temperature of the suppliedwater as well as on the parallel reaction with the SiO₂ and Al₂O₃contained in the ash mixture. Since the hydration product has a lowerdensity than the sorbent, this reaction causes the sorbent grain to“swell,” so that the adsorbate blanket (or gypsum blanket) around thesorbent aggregate gets broken up.

[0025] In this connection it has turned out that the degree ofconversion of the SO₂ sorbent into hydration product as well as thereaction rate increase considerably with increasing temperature, andwithin the range of about 60° C. to about 100° C. and at atmosphericpressure they reach a value that is optimally desirable in such anoperation. If the temperature is too low, then the formation ofhydration product proceeds only very slowly and is not complete withinthe dwell time that is available in the mixing unit. If the temperatureis too high, then a “boiling” of the ash mixture takes place, with theconsequence that due to the excess reaction enthalpy, additional wateris evaporated, which is then no longer available for the hydrationreaction. The consequences of reaction temperatures that are too highare an increased water consumption and problems associated with theadditional vapor formation.

[0026] The reaction temperature in the mixing unit is regulated in anexpedient manner in order to achieve an optimal conversion of unreactedSO₂ sorbent into a hydration product. This enables the dosed-outquantity of water or of sodium solution to be adjusted in accordancewith the concentration of the unreacted SO₂ sorbent in the ash. Thus viaa thermodynamic balance the optimal quantity of water to be added can bedetermined.

[0027] In another advantageous embodiment of the invention, the desiredreaction temperature in the mixing unit is achieved by adding preheatedwater into the mixing unit, with the water temperature being regulatedby a preheater located in front of the mixing unit. By the preheating ofthe water, the reaction temperature in the mixing unit can be set withinthe temperature range that is favorable to the course of the reaction,independently of the concentration of the non-reacted sorbent in theash. Instead of water, an aqueous, sodium-containing solution can alsobe supplied.

[0028] Through the regulation of the quantity of water or of aqueoussodium-containing solution that is fed to the mixing unit, the mixtureproduct can be carried off from the mixing unit in an advantageousmanner as a function of the residual moisture. In this way, the productthat is to be carried off can be produced in a desirable fashion.

[0029] In one advantageous embodiment of the invention, the dwell timeof the product introduced into the mixing unit is regulated as afunction of the degree of hydration of the product to be carried off. Itis especially advantageous when the minimum dwell time in the mixingunit and/or the subsequent delivery lines amounts to one minute, inorder to ensure that the hydration of the introduced product occurs inthe desired manner.

[0030] It is expedient for the product drawn off from the mixing unit toexist in the form of a solid and to have a residual moisture less than10%. This keeps the product drawn off from the mixing unit from beingsludgy and thus difficult to transport due to a too-high excess ofwater.

[0031] It may be advantageous to construct the mixing unit in twostages, where in the first stage a portion of the water or of aqueous,sodium-containing solution required for the mixing is admixed with theash, the reaction product, and the unreacted sorbent, and in the secondstage the remaining portion of water or aqueous, sodium-containingsolution is admixed in a regulated way as a function of the residualmoisture of the product to be carried off from the mixing unit. In thisway, in the first mixer attention can be directed toward the mixingprocess and in the second mixer it can be directed toward the requisitedwell time as well as to the temperature requirements.

[0032] By feeding the product carried off from the mixing unit into adrier, this product can be stored in an advantageous way after it hasbeen dried. Thereby, in another advantageous embodiment of the inventionthe possibility is provided of putting this product into intermediatestorage and feeding it to the combustion chamber after a certaininterval of time.

[0033] It is further advantageous to return at least a portion of theproduct carried off from the mixing unit back into the mixing unit. Withthis measure, the dwell time for the reaction within the mixing unit canlikewise be affected.

[0034] By feeding the ash mixture into a sifting/sizing unit beforeintroducing it into the mixing unit, grain sizes of undesirablemagnitude, for example larger than 300 microns, can be sifted out. Thismakes it possible to largely avoid erosions within the mixing unit aswell as in the delivery lines. The same effect can be achieved bydirecting relatively large and undesirable grain sizes into a grindingunit and subsequently passing these on to the mixing unit.

[0035] In one advantageous embodiment of the invention, the ash, thereaction product, and the unreacted SO₂ sorbent drawn off from thecirculating fluidized-bed firing system and fed to the mixing unit issupplied to the mixing unit from the flue-gas filter as filter ash andfrom the combustion chamber as bottom ash over separate supply lines ineach case, in which connection either filter ash or bottom ash or anadjustable mixture of the two can be fed to the mixing unit. This makesit possible to respond to any operational situation of the fluidized-bedfiring system.

[0036] Furthermore the apportionment of the mixture of filter ash andbottom ash can be set by adjusting the grain sizes of added fuel and SO₂sorbent.

[0037] It is advantageous to introduce the water or an aqueous,sodium-containing solution into the mixing unit by means of at least onenozzle. As the situation demands, the requisite nozzles can be installedwithin the mixing unit at any points desired.

[0038] It is advantageous to use limestone as the SO₂ sorbent. This isrelatively inexpensive and has worked well as an SO₂ sorbent.Furthermore it may also be advantageous to use dolomite as the SO₂sorbent.

[0039] In another advantageous embodiment of the invention, 50% to 500%of solid mixture, relative to the solid mixture normally leaving thecombustion chamber of the fluidized-bed firing system, is fed to themixing unit for the hydration process and subsequently fed again to thecombustion chamber. Thereby an optimal conservation of the needed SO₂sorbent is achieved.

[0040] In one special case of the procedure in accordance with theinvention, a portion of the SO₂ sorbent is delivered directly to themixing unit. In this way, regardless of the amount of ash in circulationas well as unreacted SO₂ sorbent, the requisite amount of SO₂ sorbentcan be influenced from the outside.

[0041] In another special case of the procedure in accordance with theinvention, a hydration product or Ca(OH)₂ can be delivered into the linebetween the mixing unit and combustion chamber, with it beingadvantageous for this product to be delivered upstream of anintermediate storage, as seen in the direction of flow of the product.In this way, this quantity of product can likewise be influenced fromthe outside.

[0042] If an aqueous, sodium-containing solution is used, it isadvantageous for the solution to contain sodium in the form of ions onan order of magnitude of up to 3% by weight, relative to the unreactedSO₂ sorbent.

[0043] Furthermore it may be advantageous for the mixing unit to includean additional feed-in of water or of an aqueous, sodium-containingsolution, by means of which water or an aqueous, sodium-containingsolution is added to the ash, the reaction product, and the unreacted S0₂ sorbent upstream of the mixing unit. Thereby the reaction in themixing unit can be influenced beforehand if appropriate.

[0044] Through the apparatus in accordance with the invention, theprocedure in accordance with the invention can be carried out in anefficient, inexpensive, and resource-sparing manner.

BRIEF DESCRIPTION OF THE DRAWINGS

[0045] The present invention may be better understood and its numerousobjects and advantages will become apparent to those skilled in the artby reference to the accompanying drawings in which:

[0046]FIG. 1 is a schematic diagram of a first embodiment an apparatusfor cleaning flue gas in accordance with the invention.

[0047]FIG. 2 is a schematic diagram of a first embodiment an apparatusfor cleaning flue gas in accordance with the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0048]FIG. 1 shows in a schematic way the apparatus in accordance withthe invention for the cleaning of flue gases containing sulfur dioxidethat come from fluidized-bed firing systems. By means of the apparatusshown, the procedure in accordance with the invention can also becarried out.

[0049] The circulating fluidized-bed firing system 1 in accordance withthe invention has a fluidized-bed combustion chamber 2, in which in afamiliar manner coal or other particulate and carbon-containing fuelsare burned by the addition of air—as a fluidizing medium via the line 5as well as secondary air—at 700 to 950° C. in the fluidized bed. For thedesulfurization of the sulfur-containing fuel, in addition to the fuel(supply line 3) also an SO₂ sorbent is delivered to the combustionchamber 2 via the line 4. As a rule, limestone in the form of particlesor fine grains, namely CaCO₃, is delivered to the combustion chamber 2as the SO₂ sorbent. In place of limestone, also dolomite as well asburnt lime (CaO) or Ca(OH)₂ can be used as an SO₂ sorbent.

[0050] When limestone is used as the SO₂ sorbent, first of all theprocess of roasting the limestone into burnt lime (CaO) takes place inthe combustion chamber 2 and subsequently a reaction occurs between theroasted limestone and the sulfur dioxide of the flue gas that has arisenin the combustion of the sulfur-containing fuel. In the temperature thatprevails in combustion chamber 2, carbon dioxide is expelled from thelimestone and burnt lime remains behind, which because of the expulsionof the CO₂ is highly porous and thus has a high specific surface area.In the subsequent gas-solid reaction of the burnt lime as an SO₂ sorbentwith sulfur dioxide and oxygen, the latter are adsorbed on the surfaceand in the pores of the sorbent, and gypsum (CaSO₄) remains behind as areaction product. Here the abovementioned high specific surface area ofthe SO₂ sorbent is extremely important to its having a high absorbingcapacity or reactivity.

[0051] Depending on the grain size of the limestone or SO₂ sorbent usedand its abrasion properties, either the sorbent-reaction productaggregate (lime-gypsum aggregate) remains in the combustion chamberuntil it is drawn off via the components or media 11 of thecombustion-chamber ash removal system, or in the case of small particlesthe sorbent-reaction product aggregate leaves the combustion chamber 2together with the flue-gas stream through the line 8, and after passingthrough a separator 6 (particles separated out in the separator 6 arerecycled to the combustion chamber 2 through the line 22) and through asteam generator or heat-recovery adjuncts 7, to which a large portion ofthe heat contained in the flue gas is released, it is subsequentlyseparated out in the following flue-gas filter 9.

[0052] Here the mixture of ash, reaction product or gypsum, and free,non-reacted limestone, which is drawn off via the combustion-chamber ashremoval system, is usually designated as coarse or also as bottom ash,and the mixture that is separated out in filter 9 is designated asfilter ash. In what follows, these designations are used for theabovementioned mixtures.

[0053] The bottom ash and the filter ash thus arrive via two separatecircuits—the bottom ash via the line 11 and the filter ash separated outfrom the flue gas in the flue-gas filter 9 via the line 8 (jointly withthe flue gas) and line 10—from the combustion chamber 2 into amechanical mixing unit 12. In this mixing unit 12, water is fed to theabove-mentioned mixture through a supply line 14 and via at least onefeed nozzle 23 for their mixing together. In case it is necessary, alsoa number of feed nozzles 23 can be positioned in the mixing unit 12 viathe product-passage segment. The mixing unit 12 can furthermore includeupstream of the mixing unit 12, in the line 10 and/or 11, another feednozzle 23. By means of this nozzle, the mixing process can beinfluenced.

[0054] In accordance with the invention, the addition of water into themixing unit 12 takes place at a reaction temperature of 60° C. to 100°C. and at atmospheric pressure (about 1 bar). In such a case the stillfree, namely unreacted burnt lime reacts within the framework of anexothermic reaction to form hydrated lime (hydration product), with thereaction rate and the temperature elevation depending on the free-limeconcentration of the ash, the temperature of the added ash, as well asthe temperature of the supplied water. Because of the lower density ofhydrated lime compared to lime, the reaction causes the lime aggregateto swell, so that the gypsum blanket around the lime aggregate is brokenup and the gypsum blanket becomes permeable, and thereby the poroussurface of the lime aggregate regains at least partially its SO₂sorbability.

[0055] Instead of water, also an aqueous, sodium-containing solution canbe fed to the product mixture in the mixing unit 12. Due to the sodiumcontained in the solution, the surface temperature of the reactionproduct is lowered and on its surface a liquid phase forms, whichimproves the SO₂ sorbability of the reaction product in connection withthe reaction of sodium with SO₂ (Na₂SO₄).

[0056] The requisite reaction temperature in the mixing unit 12 can beregulated, with it being advantageous for the water or the aqueous,sodium-containing solution fed to the mixing unit 12 through line 14 tobe preheated in accordance with the required reaction temperature. Thishappens in a preheater 15 positioned in the line 14, which depending onthe required reaction temperature in the mixing unit 12 is regulated bymeans of a heat regulating mechanism 16. It is furthermore possible toregulate the amount of water or aqueous, sodium-containing solution fedto the mixing unit 12 as a function of the residual moisture of theproduct to be carried off from the mixing unit 12. It is advantageousfor this to take place by means of an appropriate driving of theregulating valves 24 that lie upstream, on the side of the flow media,of the nozzle or nozzles 23 positioned at the mixing unit 12.

[0057] With respect to the product to be carried off from the mixingunit 12, in order to achieve the degree of hydration for this that iswithin a requisite range, the dwell time of the product introduced intothe mixing unit 12 can also be regulated. This can be done, for example,by regulating the speed of transport in the mixing unit 12. It isexpedient for the minimum dwell time to amount to one minute for theproduct that is to be mixed together and is to undergo a reaction withinthe mixing unit 12 and possibly also within the subsequent deliverylines.

[0058] A return line 19 makes it possible for a portion of the productcarried off from the mixing unit 12 to be delivered again to the mixingunit 12 for a further or a renewed reaction and thus this makes itpossible to influence the dwell time as well.

[0059] In addition to the reaction in the mixing unit 12 that preferablyproceeds under atmospheric pressure and at a reaction temperature of 60°C. to 100° C., the reaction within the mixing unit 12 can also becarried out under a higher pressure. For example, the reaction can bebrought about under a pressure of 5 bars and a reaction temperatureequal to or less than 151° C., namely just below the boiling temperatureassociated with this pressure. The mixing unit 12 is constructed to bepressure-tight to the extent adequate to the chosen pressure.

[0060] In one useful embodiment of the invention, plowshare or paddlemixers are used as the mixing unit 12 for mixing the product mixturetogether with the water or aqueous, sodium-containing solution. However,agitators can also be used, if appropriate.

[0061] In one embodiment of the invention, by feeding the coarse orbottom ash and the filter ash in two separate lines or circuits theproportion of the bottom ash or filter ash that is supplied each timecan be varied. For example, five quantitative shares of ground ash andone quantitative share of filter ash can be fed to the mixing unit 12.This makes it possible to have an operating mode for the mixing unit 12that can be coordinated with the quantities of bottom ash or filter asharising from the combustion chamber 2.

[0062] Through the use of the procedure in accordance with the inventionin a circulating fluidized-bed firing system its advantages becomeclear, since here we have a close regulation of the combustion-chambertemperature, a long dwell time for the solid particles that are to beburned and that are introduced as SO₂ sorbent, a high degree ofgas/solid particle intermixing, and as a result of this a low Ca/Sratio.

[0063]FIG. 2 shows a two-stage mixing unit 12, where in the first stage12′ a portion of the water or of an aqueous, sodium-containing solutionrequired for the mixing is admixed with the ash, the gypsum aggregate,and the unreacted burnt lime, and in the second stage 12″ subsequent tothe first stage 12′ the remaining portion of the water or of the aqueoussodium-containing solution is admixed in a regulated fashion as afunction of the residual moisture of the product to be carried off fromthe mixing unit 12.

[0064] It is advantageous for the ash, the gypsum, and the hydrationproduct carried off from the mixing unit 12 to exist in the form of asolid and to have a residual moisture of less than 10%. Furthermore theproduct carried off from the mixing unit 12 can be fed to a drier 20, inwhich the product can be dried, for example, into a product capable ofbeing stored and if appropriate can be stored in an intermediate store21 and can be fed to the fluidized-bed combustion chamber 2 after acertain interval of time.

[0065] By means of the line 13 the ash, the gypsum (reaction product)and the hydrated lime (hydration product) is fed from the mixing unit 12to the combustion chamber 2, in which there occurs, because of thetemperatures prevailing there, a counter-reaction and a splitting off ofwater vapor from the hydrated lime. Due to the thermal stress,additional cracking develops in the lime aggregate (reaction product),whereby the specific surface area is increased further. Due to thebreaking up of the gypsum blanket as well as through the reactionsurface additionally created by the swelling of the lime core, the limethat has not yet undergone a reaction in the interior of the previouslyblanketed or partially blanketed lime grains, namely the SO₂ sorbent,becomes available again for a more extensive desulfurization reaction.

[0066] The procedure in accordance with the invention is used especiallyefficiently when the concentration of non-reacted SO₂ sorbent in thecombustion chamber 2 amounts to 5 to 40 percent by weight of the solidmixture carried off from the combustion chamber 2.

[0067] Before conducting the ash, the reaction product gypsum, and theunreacted lime into the mixing unit 12, this product can be sifted/sizedand/or ground in units 17, 18 suitable for this purpose, in order toobtain the best possible conditions for the mixing process. In thisconnection it is expedient to use a sifter with a mesh width such thatparticles larger than 300 microns are sifted out and are separated outof the circuit. This has the advantage of preventing major erosions fromoccurring in the mixing unit 12 as well as the hydrating of stonymaterials.

[0068] The apparatus in accordance with the invention and the procedurein accordance with the invention allows either combustion-chamber bottomash or filter ash or a mixture of the two to be fed to the mixing unit12. In this connection, it is advantageous for the apportionment of themixture of filter ash and bottom ash to be set by adjusting the grainsizes of added fuel and limestone.

[0069] In the procedure in accordance with the invention, there is asavings of 20% to 40% of SO₂ sorbent, for example limestone, as comparedto previously familiar procedures. When compared to a customary sulfurretention of 95% and a customary Ca/S ratio (the ratio of SO₂ sorbent tosulfur that is to be retained) of 2, in the procedure in accordance withthe invention and with a sulfur retention of 95% the result is a Ca/Sratio of 1.2. Thus this value already lies very close to the idealstoichiometry of 1.0. With that, through the procedure in accordancewith the invention considerably less SO₂ sorbent is needed for theretention of the sulfur present in the flue gas. This has a substantialeffect on the operating costs of the system, especially when fuel havinga high sulfur content is used.

[0070] The transporting of the product in the lines 10, 11, 13, or 19can be done, for example, by either pneumatic or mechanical means.

[0071] The supply line 25 makes it possible to deliver more SO₂ sorbentto the mixing unit 12. Thereby, regardless of the concentration ofunreacted SO₂ sorbent in the mixture delivered to the mixing unit 12,this mixture can be enriched by means of an externally supplied SO₂sorbent. Likewise, by means of the line 26 the hydration product Ca(OH)₂can be supplied to the line 13, which leads from the mixing unit 12 tothe combustion chamber 2. By means of this measure, it is possible toinfluence the quantity of hydration medium, namely the medium that isreactivated again into an S02 sorbent in the combustion chamber 2.

[0072] In one advantageous embodiment of the invention, 50 to 500% byweight of the solid mixture, relative to the solid mixture thatcustomarily leaves the combustion chamber 2 of the fluidized-bed firingsystem 1, is supplied to the mixing unit 2 for the hydration process andis subsequently supplied again to the combustion chamber 2.

[0073] In the procedure according to the invention, usually a continuousprocess is involved, but in special applications it is also possible tohave a discontinuous mixing together and enrichment of the ash mixturewith water in the mixing unit. In this case the hydration product iseither discontinuously supplied to the combustion chamber 2 or thehydration product stored in an intermediate store 21 is continuouslydrawn off from this intermediate storage 21 and supplied to thecombustion chamber 2.

[0074] While preferred embodiments have been shown and described,various modifications and substitutions may be made thereto withoutdeparting from the spirit and scope of the invention. Accordingly, it isto be understood that the present invention has been described by way ofillustration and not limitation.

What is claimed is:
 1. Process for the cleaning of flue gases containingsulfur dioxide produced by sulfur-containing coal which is burned by theaddition of air at a temperature of 700 to 950° C. in the combustionchamber of a circulating fluidized-bed firing system, the processcomprising the steps of: delivering a particulate SO₂ sorbent to thecombustion chamber; feeding a portion of the ash produced in thecombustion and a portion of the reaction product produced in thereaction of the SO₂ sorbent with the sulfur dioxide as well as a portionof the unreacted SO₂ sorbent from the combustion chamber to a mixingunit; supplying water or an aqueous sodium-containing solution to themixing unit and mixing the ash, the reaction product, the unreacted SO₂sorbent together with the water or aqueous sodium-containing solution,whereby the unreacted SO₂ sorbent is converted into a hydration productat a reaction temperature of 60° C. to 100° and at atmospheric pressure;returning the ash, the reaction product, and hydration product from themixing unit into the combustion chamber of the fluidized-bed firingsystem; and reactivating the hydration product into an SO₂ sorbent at acombustion-chamber temperature of 700° to 950° C.
 2. Process accordingto claim 1 further comprising the step of supplying water or an aqueoussodium-containing solution to the ash, the reaction product, and theunreacted SO₂ sorbent upstream of the mixing unit.
 3. Process accordingto claim 1 further comprising the step of regulating the reactiontemperature of the mixing unit.
 4. Process according to claim 3 whereinthe step of regulating the reaction temperature of the mixing unitcomprises the sub-step of preheating the water or the aqueoussodium-containing solution fed to the mixing unit.
 5. Process accordingto claim 1 further comprising the step of regulating the water oraqueous sodium-containing solution fed to the mixing unit as a functionof the residual moisture of the product to be carried off from themixing unit.
 6. Process according to claim 1 further comprising the stepof regulating the dwell time of the products introduced into the mixingunit as a function of the degree of hydration of the product to becarried off.
 7. Process according to claim 1 further comprisingmaintaining a minimum dwell time of the products introduced into themixing unit amounts of 1 minute in the mixing unit and the subsequentdelivery lines.
 8. Process according to claim 1 wherein the mixing unithas first and second stages and the step of supplying water or anaqueous sodium-containing solution to the mixing unit comprises thesub-steps of: mixing a first portion of the water or aqueoussodium-containing solution with the ash, the reaction product, and theunreacted SO₂ sorbent in the first stage of the mixing unit to form amixture; mixing a second portion of the water or aqueoussodium-containing solution with the mixture from the first stage of themixing unit in the second state of the mixing unit; and regulating themixing of the second portion of the water or aqueous sodium-containingsolution with the mixture from the first stage of the mixing unit in thesecond state of the mixing unit as a function of the residual moistureof the product to be carried off from the mixing unit.
 9. Processaccording to claim 1 wherein the ash, the reaction product, and thehydration product returned to the fluidized bed firing system from themixing unit is in a solid form and has a residual moisture of less than10%.
 10. Process according to claim 9 wherein the step of returning theash, the reaction product, and hydration product from the mixing unitcomprises the sub-steps of: feeding the ash, the reaction product, andhydration product from the mixing unit into a drier; and drying the ash,the reaction product, and hydration product into a product capable ofbeing stored.
 11. Process according to claim 10 wherein the step ofreturning the ash, the reaction product, and hydration product from themixing unit further comprises the sub-steps of: storing the ash, thereaction product, and the hydration product in an intermediate store;and feeding the ash, the reaction product, and the hydration productfrom the intermediate store to the fluidized-bed combustion chamberafter a predetermined time interval.
 12. Process according to claim 1further comprising the step of feeding at least a portion of the ash,the reaction product, and the hydration product returned from the mixingunit back to the mixing unit.
 13. Process according to claim 1 furthercomprising the step of sifting and sizing the ash, the reaction product,and the unreacted SO₂ sorbent before it is fed from the combustionchamber to the mixing unit.
 14. Process according to claim 1 furthercomprising the step of grinding the ash, the reaction product, and theunreacted SO₂ sorbent before it is fed from the combustion chamber tothe mixing unit.
 15. Process according to claim 1 wherein thecirculating fluidized-bed firing system also includes a flue-gas filter,a first supply line providing fluid communications between the mixingunit and the flue-gas filter, and a second supply line providing fluidcommunication between the mixing unit and the combustion chamber, andthe step of feeding the ash, reaction product, and unreacted SO₂ sorbentcomprises feeding an ash, reaction product, and unreacted SO₂ sorbentmixture selected from the group consisting of filter ash from theflue-gas filter, bottom ash from the combustion chamber, and anadjustable mixture of filter ash and bottom ash.
 16. Process accordingto claim 15 further comprising the step of adjusting the grain sizes ofadded fuel and SO₂ sorbent to apportion the mixture of filter ash andbottom ash.
 17. Process according to claim 1 wherein the water oraqueous sodium-containing solution is introduced into the mixing unit byway of at least one nozzle.
 18. Process according to claim 1 wherein theconcentration of non-reacted SO₂ sorbent in the combustion chamber is 5to 40% by weight of the mixture of solids carried off from thecombustion chamber.
 19. Process according to claim 1 wherein limestoneis used as the SO₂ sorbent.
 20. Process according to claim 1 whereindolomite is used as the SO₂ sorbent.
 21. Process according to claim 1wherein the aqueous sodium-containing solution contains sodium in theform of ions on an order of magnitude of up to 3% by weight, relative tothe non-reacted SO₂ sorbent.
 22. Process according to claim 1 furthercomprising the step of delivering at least a portion of the SO₂ sorbentdirectly to the mixing unit.
 23. Process according to one claim 1further comprising the step of delivering Ca(OH)₂ into the line betweenthe mixing unit and combustion chamber.
 24. Process according to claim23 wherein Ca(OH)₂ is delivered upstream of an intermediate store, seenin the direction of flow of the product flowing in the line between themixing unit and the combustion chamber.
 25. Process according to claim 1wherein 50 to 500% by weight of the mixture of solids, relative to themixture of solids normally leaving the combustion chamber of thefluidized-bed firing system, is fed to the mixing unit for hydration andsubsequently fed again to the combustion chamber.
 26. Apparatus for thecleaning of flue gases containing ash and sulfur dioxide produced byburning sulfur-containing coal in the combustion chamber of acirculating fluidized-bed firing system by the addition of air at atemperature of 700° to 950° C., the apparatus comprising: means fordelivering a particulate SO₂ sorbent into the combustion chamber, aportion of the S02 sorbent and S0 ₂ producing a reaction product, aportion of the S0 ₂ sorbent remaining unreacted; a mixing unit; meansfor feeding a mixture comprising a portion of the ash, a portion of thereaction product, and a portion of the unreacted S0 ₂ sorbent from thecombustion chamber to the mixing unit; means for supplying water or anaqueous sodium-containing solution to the mixing unit, the water oraqueous sodium-containing solution mixing together with the mixture ofash, reaction product, and unreacted SO₂ sorbent at a reactiontemperature of 60° to 100° and at atmospheric pressure, whereby theunreacted SO₂ sorbent is converted into a hydration product; and meansfor returning the ash, the reaction product, and the hydration productfrom the mixing unit into the combustion chamber; wherein in thecombustion chamber the hydration product is reactivated into an SO₂sorbent at a combustion temperature of 700° to 950° C.
 27. Apparatusaccording to claim 26 further comprising means for supplying water orfor an aqueous sodium-containing solution intermediate the combustionchamber and the mixing unit.
 28. Apparatus according to claim 26 furthercomprising means for regulating the reaction temperature of the mixingunit.
 29. Apparatus according to claim 26 wherein the mixing unitincludes first and second stages, the first stage receiving and mixingthe mixture of ash, reaction product, and unreacted SO₂ sorbent with afirst portion of the water or an aqueous sodium-containing solution andthe second stage receiving and mixing the mixture from the first stagewith a second portion of the water or an aqueous sodium-containingsolution, the mixing of the second portion of the water or an aqueoussodium-containing solution with the mixture from the first stage of themixing unit being regulated as a function of the residual moisture ofthe product that is to be carried off from the mixing unit. 30.Apparatus according to claim 26 further comprising means for regulatingthe dwell time of the products introduced into the mixing unit as afunction of the degree of hydration of the product to be carried off.31. Apparatus according to claim 26 further comprising means for siftingor sizing the mixture of ash, reaction product, and unreacted SO₂sorbent disposed intermediate the combustion chamber and the mixingunit.
 32. Apparatus according to claim 26 further comprising means forgrinding the mixture of ash, reaction product, and unreacted SO₂ sorbentdisposed intermediate the combustion chamber and the mixing unit. 33.Apparatus according to claim 26 further comprising means for drying theash, the reaction product, and the hydration product disposedintermediate the mixing unit and the combustion chamber.
 34. Apparatusaccording to claim 33 further comprising an intermediate store for thestorage of the ash, the reaction product, and the hydration productdisposed intermediate the means for drying and the combustion chamber.35. Apparatus according to claim 26 wherein the mixing unit includes atleast one nozzle for the supplying of the water or aqueoussodium-containing solution.
 36. Apparatus according to claim 26 whereinmixing unit is selected from the group consisting of a plowshare, apaddle mixer and an agitator.